B cells are selected as subsets defined by the presence (or absence) of the following membrane molecules: Ig, Ia, FcR and lectin responsive glycoproteins, and defined by the density of these glycoproteins on the cell surface. The subsets are stimulated by viral and tumor antigens, whole virus, and tumor cells and by several lectins that bind and may be mitogenic. The response of the B cell subsets to these stimuli is measured in terms of 3HTdr uptake and blastogenesis, Ca2 ion fluxes, antibody formation, and cooperation with T cells and macrophages. B cells from young aged and tumor-bearing rats are to be examined by the above procedures.